This invention relates to a recording/reproducing apparatus advantageously employed for a magneto-resistive head, referred to herein as an MR head. More particularly, it relates to a recording/reproducing apparatus for the MR head designed for reducing the recording/reproducing switching time interval.
The MR head has hitherto been employed, besides the usual induction head, as a playback head for a hard disc drive (HDD). The MR head is designed so that its thin magnetic film is changed in resistivity under the effect of a magnetic field from a magnetic medium, the change in resistivity being detected as a playback output voltage. The MR head exhibits a high output and a low crosstalk and is free from velocity dependency so that it is widely employed as a head for high density recording/playback for e.g. a digital/audio tape recorder.
Since the MR head is a playback head, it is stacked on or placed side by side with an induction thin film type recording head on one and the same substrate, or is integrated with an independent recording head, if the MR head is to be used as a recording/playback head.
Among a variety of different constructions of the MR head, there is known a shield type MR head shown for example in FIG. 20.
The shield type magnetic head, shown in FIG. 20, has an MR device 13, placed within a gap 12 defined between a pair of shield cores 11, and connected as one to a signal conductor 14. A bias conductor 15, placed side by side with the signal conductor 14, is also arranged within the gap 12. A signal magnetic field from a magnetic medium is directly picked up by the MR device 13.
Besides the above-described current bias type MR head, there is also known an MR head which is not in need of the bias conductor 15, such as a shunt bias type MR head. With such MR head, the magnetic field is generated by the MR current itself flowing through the signal conductor 14.
The construction of the playback head for the above-mentioned current bias type MR head 1 is shown in FIG. 21, in which the MR current is caused to flow through the MR device 13 from a current source 2, and changes in resistance of the MR device 13 caused by the signal magnetic field from the magnetic medium are taken out as a voltage, while the bias current is caused to flow through the bias conductor 15 from a bias current source 16 for applying the bias magnetic field across the MR device 13 for producing a linear operation of the MR device 13.
One end of the bias conductor 15 and of the MR device 13 are grounded as shown, and the voltage from the MR device 13 is supplied via a direct current blocking capacitor 3 to a playback amplifier 4 for amplification as an unbalanced output.
The capacitance of the direct current blocking capacitor 3 is selected substantially in a range of from 0.01 .mu.F to 0.1 .mu.F, depending on the bit rate, in order to allow the passage of an input signal in such an amount as not to lower the error rate.
The shunt bias type MR head has a playback circuit which is substantially the same as that shown in FIG. 21 except that the bias current source 16 and the bias conductor 15 shown therein are not employed.
FIG. 22 shows an example of a playback circuit for the MR head.
The MR head has its MR device 13 connected to an emitter of a base-grounded transistor 22 which plays the role of a first stage amplifier. The MR device 13 is connected between the emitter and the ground of the transistor 22 which has its collector connected via a load resistor 23 to a Vcc voltage source. The collector output signal of the transistor 22 is supplied to a so-called gm amplifier 24 (voltage to current converting amplifier). This gm amplifier 24 is of a differential input type and has its non-inverting input terminal and its inverting input terminal supplied with the collector output signal voltage Vcl and with a reference voltage Vref from a reference voltage source 25, respectively. The output current of the gm amplifier 24 is supplied to a capacitor 26 (LPF capacitor). The low frequency component, above all, the dc component, in the output current is allowed to pass through a low-pass filter defined by the gm (transconductance) value of the gm amplifier 24 and the capacitance of the capacitor 26 so as to be fed back to the base of the transistor 22 of the first-stage amplifier.
The cut-off frequency fc of the low-pass filter (LPF) is determined by EQU fc=1/(2.mu.C/gm) (1)
The gm (transconductance) value of the gm amplifier 24 is maintained at a lower value for maintaining the cut-off frequency fc at a sufficiently low value of e.g. 100 kHz or less and to realize low power consumption. The capacitance of the capacitor 26 constituting the above-mentioned LPF needs to be of a larger value on the order e.g. of 0.1 .mu.F.
Although the recording circuit for the recording system in the MR head reproducing apparatus shown in FIG. 22 is not shown, a R/W (read/write) IC of the recording/playback circuit is arranged for reducing the power consumption in the R/W IC to as small a value as possible by turning either the recording mode or the playback mode, off when the other mode is turned on. To this end, the gm amplifier 24 and the playback amplifier 4 formed by an initial-stage transistor 22 in the R/W IC need to be turned on and off for the playback mode or the recording mode of the playback apparatus for the MR head as shown in FIG. 2.
FIG. 23 shows a timing waveform for the reproducing apparatus for the MR head of FIG. 22 when the operating mode of the R/W IC is changed over from playback to recording and thence again to playback. In this figure, the state in which the initial-stage transistor 22 and the gm amplifier 24 are turned on and off simultaneously.
The changeover signal produced by the R/W IC is "1" or "0" from the playback mode or the recording mode, respectively, as shown at A in FIG. 23.
In FIG. 23, a solid line and a broken line indicate respectively an output signal VC1 of the initial-stage transistor 22 and a reference voltage output signal Vref supplied to the gm amplifier 24, as shown at B in FIG. 23. In such case, the rise or decay timing of the gm amplifier 24 differs from those of the initial-stage amplifier 5, depending on the difference in the current capacity, such that the output signal of the initial-stage transistor 22 rises and decays earlier. The result is that a level difference .DELTA.V is produced between the output signal VC1 of the transistor 22 and the reference voltage output signal Vref, as shown at B in FIG. 23.
Should the level difference .DELTA.V be produced, it is detected by the gm amplifier 24 and converted into an electric current which is caused to flow through the LPF capacitor 26, so that currents IC1 and IC2 are caused to flow through the LPF capacitor 26, as shown at D in FIG. 23, by an output signal gm of the gm amplifier 24 as shown at D in FIG. 23.
When the playback mode is turned on from the recording mode, excess charges accumulated in the LPF capacitor 26 are discharged gradually after the starting of the initial-stage amplifier 22 and the gm amplifier 24 is completed, so that switching from the recording mode to the playback mode comes to a close after lapse of the recording mode/playback mode switching time interval TRW shown at D in FIG. 23. Due to the recording mode/playback mode switching time interval TRW, which depends on the capacitance of the LPF capacitor 26, having a larger value on the order of 0.1 .mu.F, as mentioned above, there results a delay of several microseconds at the minimum.
In the playback circuit for the MR head shown in FIG. 22, the initial-stage transistor 22 or the gm amplifier 24 is switched from the off-state to the on-state when the power source is turned on. If the circuit is applied to the recording/reproducing apparatus, and the power source of the playback circuit is turned off for the recording mode for decreasing the power consumption, the transistor 22 and the gm amplifier are switched between the off-state and the on-state at the time of switching from the recording state to the playback state and vice versa. At this time, the LPF capacitor 26 needs to be charged and discharged. However, discharging of the capacitor 26 having a larger capacitance on the order of 0.1 .mu.F takes a prolonged time with the result that the voltage across the capacitor 26 is stabilized only after lapse of prolonged time to deteriorate the response characteristics.
For decreasing the charging/discharging time of the capacitor 26 of the larger capacity, it may be contemplated to effect quick charging/discharging by employing a gm amplifier 24 having a larger gm value. However, if the gm value is increased, the cut-off frequency fc of the LPF is increased so that effective dc feedback is disabled.
In this consideration, the present Assignee has proposed a playback circuit for the magnetic head shown in FIG. 24.
In FIG. 24, the MR device 13, base-grounded transistor 22, load resistor 23, reference voltage source 25 and the LPF capacitor 26 are the same as those shown in FIG. 22 and hence denoted by the same reference numerals. However, in the playback circuit of FIG. 24, two gm amplifiers 27, 28 having different gm values are employed. That is, the first and second gm amplifiers 27, 28 have a smaller gm value and a larger gm value, respectively.
In the playback circuit for the magnetic head shown in FIG. 24, the collector output signal of the base-grounded transistor 22 as the initial-stage amplifier is supplied to a non-inverting terminal of the first gm amplifier 27 and to the non-inverting input terminal of the second gm amplifier 28. The reference voltage Vref from the reference voltage source 25 is supplied to the inverting input terminal of the first gm amplifier 27 and to the inverting input terminal of the second gm amplifier 28. Output signals of these gm amplifiers 27, 28 are supplied to the LPF capacitor 26.
If the playback circuit is changed from the off-state to the on-state, as when the power source is turned on, the second gm amplifier 28 having the larger capacitance value is first turned on to charge the large capacitance capacitor 26, after which the first gm amplifier 27 is turned on to derive the cut-off frequency required for LPF by the gm value of the amplifier 27 and the capacitance value of the capacitor 26.
Meanwhile, if the outputs of the gm amplifiers in the steady-state condition are equal, the value of (Vref--initial stage output), which is an offset in the output signals, becomes smaller the larger the gm value. That is, since the output offset when the first gm amplifier 27 is turned on and that when the second gm amplifier 28 is turned on differ from each other, the switching between these amplifiers 27, 28 leads to dc level fluctuations in the playback output as indicated by the graph shown in FIG. 25. In this figure, the playback circuit is changed from the off-state to the on-state at time t=0, with the second gm amplifier 28 being turned on. At time t=t0, the second gm amplifier 28 is turned off, while the first gm amplifier 27 is turned on. At such time, dc fluctuations .DELTA.V is incurred in an output of the transistor 22 operated as an initial-stage amplifier. Besides, the circuit construction is complicated because of the necessity for providing a switching controlling circuit, not shown, for changing over the two amplifiers 27, 28.